Dr Gary Fuller, of King’s College London’s Environmental Research Group, explained: “On cold days you get a cold layer of air that sits close to the ground and all of our air pollution is put into that layer, therefore the concentration goes up.

“Yesterday was one of those rare moments where air pollution was actually visible; you can’t normally see it like that.”

He added that normal air pollution levels for London are between one and three but “yesterday we had numbers which were breaking the 7, 8, 9, 10 mark.”

The figures come from the London Air Network’s ‘nowcast’ 1-10 scale, which measures levels of four toxic pollutants at locations across the city.

“Air pollution can seem intangible but there are a few moments when it is made visible and you can see it co clearly,” Dr Fuller said.

Mr Buckney, who cycles up Horniman Hill every day to drop his children at school, said it is on the clearest, brightest days that air pollution looks most severe.

The 48-year-old told the Standard: “On clear days you can really see the state of things, and yesterday it was shocking.

“I cycle up this hill every day and I love the view of London, but it is very concerning to see how bad the pollution is.

“When I can see the air pollution I avoid cycling into town and try to take the quickest routes possible, so that I don’t exert myself too much.”

Another picture posted on Twitter by quantative analyst William Smith shows a similar layer of pollution visible looking out across the O2 arena to east London.

Winds from the Atlantic were expected to ease pollution later on Friday but the Environment Department is still forecasting moderate to high pollution following yesterday’s very high peak.

Parents have also been advised to “take care” when taking their baby outside by the pioneering Plume Labs company after it recorded toxic levels of air pollution.

The firm put the level of air pollution in London as “high” based on World Health Organisation limits, which are stricter for some pollutants than EU rules used by the Government and other experts.

A spokesman for London mayor Sadiq Khan said: “Sadiq has proposed the toughest crackdown on polluting vehicles by any major city in the world.

“He is also consulting on a new emissions surcharge and introducing an expanded Ultra Low Emission Zone a year earlier than the previous Mayor planned.”

“[This week] he revealed that no more pure diesel double-deck buses will be added to the capital’s fleet from 2018 and that all new single-decks for central London will be zero-emission.

“The Mayor cannot tackle air quality on his own and he is also calling for the Government to rise up to the challenge and pass new legislation fit for the 21st century.”

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